As vehicle electronic systems are being increasingly used to implement motor vehicle functions, it is becoming ever more important to protect the stability of the vehicle electrical systems from failure and from fluctuations in the power supply system. In this context, the current and voltage supply for the consumers of a motor vehicle places special demands on the voltage regulators employed, since the vehicle system voltage fluctuates within a very broad range in dependence upon the battery charge, the vehicle operation, and the ambient temperature. The fluctuation range of the vehicle system voltage is significantly affected by the connection of heavy consumers to the load. Thus, a considerable voltage drop can occur when the engine is started. In spite of these voltage fluctuations, it is necessary to ensure a constant current and voltage supply for the consumers. A regulated voltage that is constant to the greatest possible degree is needed, in particular, for the motor-vehicle control units.
To generate a constant power supply voltage, a low-voltage detection is provided, as described, for example, in German Patent No. 198 38 003. It can be used to prevent certain voltage-critical processes, such as EEPROM memory accesses. In this context, linear regulators or switching regulators having different operational ranges can be used as voltage transformers.
German Patent Application No. 199 17 204 describes a device in which an in-phase regulator is connected in parallel to a switching (switched-mode) regulator to generate a stabilized power supply voltage. The in-phase regulator and the switching regulator are dimensioned in such a way that at lower voltages, the in-phase regulator is active in controlling the voltage supply and, at the usually present higher voltages, the switching regulator is active in controlling the supply, this switching regulator switching itself off when the voltage drops too far. In the event of failure of one of the two regulators, the two parallel-connected regulators additionally facilitate delivery of a somewhat stabilized power supply voltage.
German Patent No. 40 15 351 discusses a current-supply device in which a linear regulator and a switching regulator are connected in parallel as voltage regulators. The regulators are activated as a function of the exceedance of limiting values constituted as predefined voltage values. By setting the limiting values, different operating modes of the voltage regulator can result.
German Patent Application No. 29 33 029 describes a power supply circuit for use in transmitters and receivers, where a high-current chopper-type voltage amplifier (regulator) and a low-current linear amplifier (regulator) are driven in parallel to generate a ripple-free output during the receiving phase and a stable voltage during the transmitting phase. In the process, the linear amplifier is only activated during the receiving phase and the chopper-type regulator only during the transmitting phase. By selecting the drive circuit control in this manner, emission by the chopper-type regulator that would otherwise be interfering during the receiving phase is avoided.